thunderbolt: Use DP_LOCAL_CAP for maximum bandwidth calculation

The DisplayPort IN adapter DP_LOCAL_CAP holds the aggregated
capabilities and gets updated after graphics side does the DPRX
capabilities read so we should use this to figure out the maximum
possible bandwidth for the DisplayPort tunnel.

While there make the variable name to match better what it is used for
and add kernel-doc comment to the function.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>